The principal objective of this research program is to analyze the ultrastructural organization of chemically identified systems of axons in the median eminence of the rat. The proposal consists of three major sections. In the first portion of the proposal, the normal ultrastructural organization of the median eminence will be studied with fine structrual morphometric techniques. Four levels throughout the rostrocaudal axis of the median eminence have been chosen for the analysis on the basis of previous investigations which have demonstrated differential distribution of chemically distinct systems of axons in the median eminence. In the second portion of the program, electron microscopic immunoperoxidase techniques will be utilized to study the organization of individual systems of axons in the median eminence. Antibodies generated against the following antigens are available for these studies: vasopressin, oxytocin, somatostatin, avian pancreatic polypeptide, tyrosine hydroxylase, glutamic acid decarboxylase, luteinizing hormone releasing hormone, serotonin, B-endorphin, adrenocorticotropin releasing hormone, Alpha-melanocyte stimulating hormone, leu-enkephalin and neurotensin. In the final phase of the investigation, attempts will be made to perfect techniques which will allow localization of two or more antigens within a single ultrathin section in order that the interrelation of dense and overlapping, although chemically distinct, systems of axons can be studied.